The surgical implantation of electronic devices in humans and animals has become a commonplace occurrence. These implantable medical devices perform either observational or regulatory functions. Observational functions include the monitoring of certain body functions, such as heart rate. Regulatory functions may take the form of electrical stimulation to body tissue, nerves or organs. The cardiac pacemaker is perhaps the most commonly known implantable medical device, and is an example of one that performs both observational and regulatory functions. The pacemaker or "pacer" monitors the heart to determine whether it is beating properly, and, if not, the pacer stimulates the heart muscle to restore a regular heartbeat.
In order for an implantable device to perform its functions at minimum inconvenience and risk to the person or animal within whom it is used, such devices perform noninvasive bidirectional telemetry to allow data and commands to be passed back and forth between the implantable device and an external transceiver. The transceiver, known by a variety of names, such as controller, programmer or monitor, receives the data from the implantable device. The data may take the form of device identification information, biological data, or battery charge condition, among others. Based upon this data, the programmer can transmit commands to the implantable device to optimize performance.
A number of techniques have been used for communicating noninvasively with an implantable device. The system shown in U.S. Pat. No. 4,223,679, issued to Schulman et al., and assigned to the assignee of the present invention, illustrates one example. That telemetry system transmits information from an implanted medical device to an external programmer by relying on the reflected impedance of an internal inductive circuit in the implanted device. The internal inductive circuit is energized by an inductively coupled external inductive circuit that is located in the programmer. The implanted device modulates the internal inductive circuit using frequency shift keying. By measuring the reflected impedance of the internal inductive circuit, the external programmer receives information transmitted from the implanted device. Because the energy used to transmit the information is provided by the programmer through the external inductive circuit, the implanted device uses little or no current to perform telemetry. Unfortunately, this system has speed limitations making it unsuitable for transmitting the amount of information currently required by medical telemetry systems.
Another type of device uses an active transmitter; that is, the transmitted energy is provided by the implantable device battery rather than from external sources. Such a device is illustrated in U.S. Pat. No. 4,944,299, issued to Silvian, and assigned to the assignee of the present invention. The techniques used in active systems have achieved higher data transmission rates than those used in passive devices.
By their nature, active transmitters require more power from the implantable device than passive transmitters. The power consumed by an implantable device is one of the most, if not the most, important factor in designing such devices. Implanted systems are customarily powered by a long-lasting non-replaceable internal battery. For example, the life of a pacemaker battery averages approximately five to seven years. During normal operation, conventional pacemakers experience a current drain on the order of tens of micro amperes (.mu.A). Conventional telemetry systems can drain up to fifteen additional .mu.A from the pacer battery. When the battery voltage decreases below a prescribed amount, the entire pacer must be surgically replaced.
One other design consideration is that the information conveyed between an implanted medical device and an external programmer must be accurate and nearly error-free because of the risks to the human or animal in whom the device is implanted. During the design of the device, it is programmed to transmit a predetermined digital signal. The signal is received outside the device and compared to the predetermined transmitted signal. A measure of signal quality known as the bit error rate (BER) is then calculated according to the following formula: BER =(number of bits in the received signal that are not identical to the bits in the same bit positions in the transmitted signal)/(total number of bits in the signal). The major factors in determining this signal quality measurement include ambient electrical noise and the distance between the device and the programmer. Accurate telemetry requires a bit error rate on the order of 10.sup.4.
To ensure satisfactory telemetry during normal operation after implantation, the device transmitter is designed with enough radiated power to guarantee an acceptable error rate under worst case conditions. Conventional systems assume telemetry in an electrically noisy environment at a maximum distance of approximately four inches, and set the transmitter power level of the implantable device accordingly to maintain effective telemetry under those conditions. Typically, this level is fixed and not adjustable after the pacemaker has been implanted. However, in many circumstances after implantation, telemetry is carried out in relatively quiet environments at less than the maximum telemetry distance. Thus, the worst case is not frequently realized. Although telemetry is guaranteed under this scenario, setting power according to worst case assumptions causes unnecessary power consumption and a shortening of potential battery life.